1. Field of the Invention
Embodiments of the present invention relate generally to time and frequency alignment systems operating over communications networks and, more specifically, to methods and apparatus for precision time transfer over optical fiber.
2. Description of the Related Art
It has been recognized that synchronizing network elements in optical communications networks to a high level of precision enables the provision of advanced services. Consequently, time and frequency alignment are essential to certain types of systems operating with conventional optical networks. For example, accurate time alignment is required by cellular networks, services carried out in real time over a network, and network elements that use packet-based signal formats for multiplexing, transmission, and switching. Similarly, frequency alignment is required in time-division multiplexing (TDM) and media streaming systems that require fixed video or audio sample rates across multiple clients.
One approach known in the art that provides both time and frequency alignment involves computing an aligned time signal based on a master timing signal from a primary reference clock, such as a global positioning system (GPS) satellite timing signal, which is held in precise alignment with a global clock reference. Using GPS signals or other master timing signals at each network element to achieve time or frequency alignment is generally prohibitively expensive and requires each network element to be able to receive satellite time signals from GPS satellites. There are many situations where visibility of GPS satellites may be compromised, interfered with, or interrupted. GPS also has known timing accuracy limitations in various operating modes, such as the L1 only mode that is most common for commercial GPS timing receivers. Consequently, an alternative approach to time alignment is to transmit timing alignment information via a protocol that is operable within a given communications network.
In conventional TDM networks a physical layer method implements frequency alignment throughout the network, starting with a designated master clock system. The designated master clock system delivers frequency and/or timing information via bit-timing and/or symbol-timing information associated with downstream physical communication links. In normal operation, each network element coupled to the master clock system regenerates and distributes the master clock timing information to neighboring downstream network elements in a point-to-point fashion over the physical medium interconnecting adjacent network elements. Thus, each network element within the TDM network receives frequency and/or timing information and aligns local frequency and/or timing with an upstream clock reference, thereby enabling every network element within the TDM network to achieve frequency alignment. Provided that adequate care is taken to avoid timing loops, such a configuration has been proven to be robust. However, the timing reference transferred between elements in the TDM environment is principally a frequency reference as opposed to a time reference.
Packet-based network synchronization methods such as Precision Time Protocol (PTP) and Network Time Protocol (NTP) transfer time and frequency references using packets containing time stamps that identify the times of departure/arrival of packets. These protocols can be used to distribute timing and frequency alignment throughout a network in a point-to-point fashion similar to the way that TDM networks distribute frequency alignment, as described above. For convenience the discussion here considers PTP though essentially identical statements apply in the case of NTP and all other packet-based time transfer methods.
A PTP “grandmaster” can transfer time to a network element coupled to it, a “slave” instantiated on that network element can reconstitute the time, and then a “master” connected to the slave in the same network element can transfer time to a subsequent network element in this point-by-point fashion. PTP and NTP can also operate in a mode where the “slave” clock in a network element can communicate directly with the “master” clock system for timing purposes. In all cases, the accuracy of two-way time-transfer protocols is adversely affected by asymmetry introduced by the communications network connecting the two network elements, including asymmetry in the physical medium, asymmetry in the construction of the forward and reverse paths in the network elements, and other sources. PTP and NTP assume that transit delays between master and slave clocks are symmetric, i.e., the transfer packet delay from a master clock to a slave clock is equal to the transfer packet delay from the slave clock to the master clock. But because forward and reverse physical paths are often different in coupled network elements, they are typically not symmetric.
Networks over which two way time transfer ranging protocols such as PTP and NTP are employed often use separate fiber strands for carrying signals in each direction, i.e., slave-to-master and master-to-slave, and assembly and deployment methods of communication networks often include short lengths of cable for mounting convenience that vary the forward and reverse path lengths between network elements by an unknown amount. The asymmetry in transit delay resulting from such variation in path length can be tens of nanoseconds or much more, while the desired level of time accuracy and time stability in a fiber-optic communication network can be on the order of nanoseconds. Introduction of dispersion compensation elements can increase this error to well beyond 10 s of microseconds. Thus, the ability of PTP and NTP to accurately transfer time between network elements in a fiber-optic network is limited. Causes of asymmetry include, but are not limited to, different propagation velocities of different wavelengths on an optical fiber, different fiber strand lengths in different optical fiber strands used for forward and reverse communications, different physical forward and reverse delays on network elements before the packet time-stamping point, etc. Specifically, the asymmetry in transit delay of timing packets between slave and master clocks provides a bound to the accuracy of time transfer.